1. Field of the Invention
The present invention relates to a method of and an apparatus for thermally recording an image or the like by recording an image or like on a thermosensitive recording medium with thermal energy applied from a thermal head and thereafter heating the thermosensitive recording medium again to produce an image of stable density.
2. Description of the Related Art
There have been widely used thermal recording apparatus for applying thermal energy to a thermosensitive recording medium to record an image or the like on the thermosensitive recording medium. Particularly, a small-size thermal recording apparatus employing a thermal head as a heat source is known from Japanese Laid-Open Patent Publication No. 59-98878, for example. Thermosensitive recording mediums for directly recording images thereon in such thermal recording apparatus include leuco thermosensitive recording paper and diazo thermosensitive recording paper. Thermosensitive recording mediums for recording images thereon through thermal transfer include sublimation-transfer thermosensitive recording paper. Further, transparent thermally sensitive recording media are being developed, whereby digital images produced by electronic medical apparatus, for example ultrasonic scanners, CT scanners, X-ray apparatus and so forth, can be directly recorded onto such recording media.
Such thermosensitive recording mediums are designed not to produce color with low thermal energy so that they can be kept in stable storage. Therefore, a considerable amount of thermal energy needs to be applied to a thermosensitive recording medium in order to cause the thermosensitive recording medium to develop a desired color. As a result, the dynamic range of image information that can be recorded on the thermosensitive recording medium is reduced by a threshold of thermal energy that must be exceeded to produce color. The reduced dynamic range makes it difficult to obtain high-gradation images recorded on the thermosensitive recording medium, and imposes a considerably large burden on thermal recording apparatus to produce color on the thermosensitive recording medium.
When a certain level of thermal energy is applied to a thermosensitive recording medium, the thermosensitive recording medium produces color depending on the applied level of thermal energy to visualize a desired image on the thermosensitive recording medium. It is known that the density of a recorded image increases with time when a thermosensitive recording medium is kept in storage at normal temperature after the image has been recorded on the thermosensitive recording medium. Consequently, the density of the image a certain period of time after it is recorded is different from the density of the same image immediately after it is recorded.
In particular, in high heat conditions such as inside an automobile in the summer, even in a short period of just a few minutes, the image density can become highly increased. Further, with images produced using the aforementioned medical treatment devices, the thermal stability, and stability over time, of the recorded image density becomes an extremely vital factor.
Herein, a prior art technique exists as disclosed in Japanese Laid-Open Patent Publication No. 61-16870, in which after thermal energy has been applied to a thermally sensitive material for producing color thereon corresponding to image information, radiant heat is radiated toward the heat sensitive recording medium, for a short time period and in a non-contacting manner, whereby heat or light is absorbed by only the image-containing portions, for raising the density thereof, whilst production of color in the non-image portions is suppressed. In this case, the density of image-containing portions with developed color therein is raised, by absorbing a heat energy which exceeds the energy needed to produce color, and on the other hand, the non-image portions which lack developed color therein, do not absorb heat or light, and since the temperature rise in those parts is kept low, color development is suppressed.
In the prior art technique, as a result of the enhancement in contrast between the image and non-image portions, the image density is improved. Nevertheless, an improvement in accuracy and/or stability of preservation of the image density cannot be assured. For example, as a result of applying radiant heat to the image-containing portions, which exceeds the heat energy required for producing color, there is the fear that an image having a density which is greater than desirable could be produced. In particular, if the time at which the radiant heat is irradiated becomes prolonged, the amount of heat energy absorbed by the thermal recording medium also increases, so that unless the intensity of illumination and/or the irradiation time of the radiant heat is very tightly controlled, a stable image density cannot be obtained. Further, were the heat source to comprise a heat roller or the like, by which direct contact with the thermally sensitive medium is made, then heat energy exceeding that required for color development would likewise be applied to the non-image portions as well, causing color development in such regions; and hence, heat cannot be selectively applied to the image-containing portions only, as required of the prior art. Accordingly, the prior art technique is limited to use with a non-contacting type of heat source.